Fix #7619: nudge fast planes sooner towards their target

[TrueBrain]

Fixes #7619 (part B).

Motivation / Problem

Fast planes kept overshooting their point for state transition. #7619 has an epic image about it.

Description

For non-NewGRF planes, "count" is never above 1. So planes can
smoothly be guided to their destination. For NewGRF planes, they
can go as quick as "count" values of 20. This easily overshoots
the target. So, calculate if the plane will overshoot, and start
nudging him to the destination earlier. You won't notice this
either way, as it all happens within a single tick.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • PR must be added to API tracker